Cooled airfoil with reduced internal turn losses

ABSTRACT

A cooled airfoil  10  has an internal fluid passage  34  that includes upstream and downstream legs, such as a co-centrifugal leg  40  and a counter-centrifugal leg  42 . The legs are separated from each other by a rib  44  but are connected in series with each other. The airfoil also includes a vent passage  56  for venting fluid from the internal passage. The intake  60  to the vent passage resides in the counter-centrifugal leg. The vent at least partially counteracts the separation potential of a separation susceptible region  54  by allowing some of the coolant to vent from the passage leg  42.

TECHNICAL FIELD

This invention relates to cooled airfoils of the type used in turbineengines and particularly to a cooled airfoil with reduced turning lossesin an internal cooling passage of the airfoil.

BACKGROUND OF THE INVENTION

Turbine engines include one or more turbines for extracting energy froma stream of hot working medium gases. A typical turbine includes arotatable hub with a set of circumferentially distributed bladesprojecting radially from the hub. Each blade includes an attachment forattaching the blade to the hub. Each blade also includes an airfoil thatspans radially across a working medium flowpath from an airfoil root toan airfoil tip. A typical turbine also includes one or more arrays ofstationary vanes axially spaced from the blades. Each vane includes anairfoil that spans radially across the flowpath and a hook or otherfeature for securing the vane to a case. Because the blades and vanesoperate in a hot environment, it is common practice to provide internalcoolant passages in at least the airfoils of the blades and vanes.During engine operation, coolant flows through the internal passages toprotect the airfoils from the intense heat of the combustion gases. Thecoolant is usually relatively cool air that has been pressurized by acompressor powered by the turbine.

Some coolant passages are multi-pass passages. A multi-pass passageincludes at least two spanwisely extending legs that are chordwiselyadjacent to each other. A spanwisely extending rib separates the legsfrom each other. An elbow at the radially inner or outer ends of thelegs wraps around one extremity of the rib to connect the legs inseries.

During engine operation, a stream of coolant flows through one of thelegs (the upstream leg), through the elbow and then through the otherleg (the downstream leg). The elbow reverses the direction of coolantflow, for example from radially outwardly in the upstream leg toradially inwardly in the downstream leg. The coolant stream entering thedownstream leg is susceptible to separation from the rib. The region ofthe leg susceptible to fluid separation extends chordwisely aconsiderable distance across the downstream leg and is characterized byhigh aerodynamic losses. These losses can imperil the durability of theairfoil by restricting coolant flow and/or by reducing the pressure ofthe coolant downstream of the region of separation. An engine designercan attempt to compensate for these effects by supplying higher pressurecoolant to the passages. However such an approach may not be completelysuccessful. Moreover, because the turbine itself is the source of energyfor pressurizing the coolant, the use of higher pressure coolantdegrades engine efficiency.

The above described susceptibility to separation arises in part from theseverity of the turn from the upstream leg to the downstream leg.However other factors are also noteworthy, particularly in blades inwhich the elbow connects the radially outer ends of two legs of amulti-pass passage. One of these factors is that the geometry of manyblades restricts the space available to accommodate an elbow near theairfoil tip. As a result, an elbow connecting the radially outer ends oftwo legs is typically squared off, rather than gently rounded, at theoutside of the turn. This leads to higher pressure losses than would beexperienced if the outside of the turn were rounded. Another factorarises from the fact that the blade rotates about the engine centerline.As a result, centrifugal effects resist the flow of coolant radiallyinwardly in the downstream passage.

SUMMARY OF THE INVENTION

According to one embodiment of the invention, an airfoil has an internalfluid passage that includes upstream and downstream legs, such as aco-centrifugal leg and a counter-centrifugal leg. The legs arechordwisely separated from each other by a rib but are connected inseries with each other. The airfoil also includes a vent passage forventing fluid from the internal passage. The intake to the vent passageresides in the counter-centrifugal leg.

The foregoing and other features will become more apparent from thefollowing description of the best mode for carrying out the inventionand the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional side elevation view of a turbine blade withinternal coolant passages in the airfoil portion thereof.

FIG. 2 is a view taken in the direction 2-2 of FIG. 1.

FIG. 3 is an enlarged view of the region 3-3 of FIG. 1 showing ventpassages for moderating a region of separation and high loss in adownstream leg of a multi-pass coolant passage.

FIG. 4 is a view similar to FIG. 3 but without the vent passage.

BEST MODE FOR CARRYING OUT THE INVENTION

FIGS. 1 and 2 show a cooled turbine blade 10 for the turbine of a gasturbine engine. The blade includes an attachment 12 for securing theblade to a hub, not shown. The hub is rotatable about an enginecenterline or axis 14. The blade also includes a platform 16 and anairfoil 18. When the blade is installed in the hub, the airfoil spansradially across a working medium flowpath 20 from an airfoil root 24 toan airfoil tip 26. A notional chord line 28 (FIG. 2) extends from aleading edge 30 to a trailing edge 32 of the airfoil. Internal coolantpassages, such as multi-pass serpentine passage 34, convey coolant 38through the airfoil. The coolant protects the airfoil from the intenseheat of combustion gases G flowing axially through the flowpath 20.

The passage 34 includes a spanwisely extending upstream leg 40 and aspanwisely extending downstream leg 42 chordwisely separated from theupstream leg by a spanwisely extending rib 44. The rib is truncated toaccommodate an elbow 46 that connects the upstream and downstream legsin series flow relationship. The upstream leg 40 is a co-centrifugal legbecause the rotation of the blade about axis 14 assists the flow ofcoolant from root end of the leg toward the tip end of the leg. Thedownstream leg 42 is a counter-centrifugal leg because the rotation ofthe blade about axis 14 resists the flow of coolant from tip end of theleg toward the root.

Referring principally to FIG. 4, the downstream leg 42 has a chordwisewidth W, as measured from the rib 44 to a chordwisely neighboring rib 48that defines the opposing sidewall of the leg. The leg 42 also has ahydraulic diameter D_(H), which is defined as four times the crosssectional area A of the leg divided by its wetted perimeter P(D_(H)=4A/P) where A and P are determined at the inlet 52 to the leg. Aregion 54 susceptible to fluid separation is present on the inside ofthe turn next to the rib 44. The chordwise dimension of the separationsusceptible region increases with increasing lengthwise distance alongthe passage, and exhibits its maximum chordwise dimension at spanwiselocation M. The maximum chordwise dimension of the separation region isabout: 50% of the chordwise width W. The region 54 then diminishes inchordwise dimension with additional lengthwise distance along thepassage. The overall spanwise dimension of the illustrated separationregion 54 is about two and one half to three hydraulic diameters. Thisis a typical spanwise dimension for the separation region, however thespanwise dimension can vary depending on the geometry of the passage legand the fluid properties of the coolant.

Referring now to FIGS. 2 and 3, a vent passage 56 penetrates the suctionsidewall 58 of the airfoil. The vent passage has an intake 60 residingin the region susceptible to separation and an exit 62 on the suctionwall. The vent passage at least partially counteracts the separationpotential of region 54 by allowing some of the coolant to vent from thepassage leg 42. We believe that the vent passage will be most effectiveif its inlet 60 resides immediately adjacent to rib 44 as seen best inFIG. 3. However, we have concluded that a vent intake chordwisely spacedfrom the rib by as much as about half the maximum chordwise width of theunmoderated region 54 (FIG. 4) would also be quite effective. Becausethe widest portion of the unmoderated separation zone (seen in FIG. 4)occupies about 50% of the local passage width W, such a vent intakewould be spaced from rib 44 by about 25% of the local passage width W.In addition, because the separation region typically extends lengthwiseabout two and one half to three hydraulic diameters lengthwise from theinlet 52, the vent intake should also be within about two and one halfto three hydraulic diameters from the inlet. Ideally, the intake is atthe lengthwise location M, where the chordwise dimension of theunmoderated separation susceptible region is widest.

The vent passage may be a single passage or it may be an array ofpassages, one example of which is the single linear array, seen in FIG.3. The above described principles for positioning the passage intakesapply equally to a single passage or to an array of passages. In thecase of a spanwisely extending row of passages, the row is centered atspanwise location M where the unmoderated separation susceptible regionhas its maximum chordwise dimension.

The vent passages may be installed by any suitable technique, forexample laser drilling, electron beam drilling or electro-dischargemachining. Since turbine blades are usually cast, the passages may alsobe provided for in the casting itself. One possible advantage to castpassages is the relative ease with which they may be precisely andrepeatably positioned in a group of serially produced airfoils.

The particular vent passage shown in the illustrations is a film coolinghole that exhausts some of the coolant 38 to the surface of the suctionwall 58 where it spreads out to form a thermally protective cooling filmon the wall surface. Alternatively a film cooling hole that ventscoolant to the surface of the pressure wall 64 would also be effective,provided the pressure difference across the passage is large enough todrive the coolant through the passage. Or, film cooling holes venting toboth the suction and pressure sides would also be effective. Ventpassages that do not also serve as film cooling holes will also sufficeto moderate the region of separation.

Although this application has shown and described a specific embodimentof our airfoil, it will be understood by those skilled in the art thatvarious changes in form and detail may be made without departing fromthe invention as set forth in the accompanying claims.

1. An airfoil having an internal fluid passage, the passage including aco-centrifugal leg and a counter-centrifugal leg in series with theco-centrifugal leg and separated therefrom by a rib, the airfoil alsoincluding a vent passage with an intake for venting fluid from theinternal passage, the intake residing in the counter-centrifugal leg. 2.The airfoil of claim 1 wherein the intake resides in a regionsusceptible to fluid separation.
 3. The airfoil of claim 1 wherein thecounter-centrifugal leg has a width and the intake resides in a regionsusceptible to fluid separation extending chordwisely up to about 50% ofthe width of the counter-centrifugal leg as measured from the rib to anopposing sidewall of the counter-centrifugal leg.
 4. The airfoil ofclaim 1 wherein the counter-centrifugal leg has a width and the intakeis spaced from the rib by about 25% of the width as measured from therib to an opposing sidewall of the counter-centrifugal leg.
 5. Theairfoil of claim 1 wherein the intake resides immediately adjacent tothe rib.
 6. The airfoil of claim 1 wherein the intake resides in aregion susceptible to fluid separation, the region extending about threeand one half hydraulic diameters lengthwise along thecounter-centrifugal leg.
 7. The airfoil of claim 1 wherein thecounter-centrifugal leg has an inlet and the intake resides within aregion extending about three and one half hydraulic diameters lengthwisefrom the inlet.
 8. The airfoil of claim 1 wherein the intake resideslengthwisely at a location where a separation susceptible region has amaximum chordwise dimension.
 9. The airfoil of claim 1 wherein theintake is an array of intakes centered at a location where a separationsusceptible region has a maximum chordwise dimension.
 10. The airfoil ofclaim 1 wherein the vent passage is a film cooling hole.
 11. The airfoilof claim 1 wherein the vent exhausts to a suction side of the blade.